Received signal quality or strength can be critical for system performances in a mobile terminal of a wireless telecommunications network. In particular the accuracy of the measurements of signal quality or strength can be critical in a system which implements closed loop power control (also known as inner loop power control) to control the power of the transmitted signal from the network to the mobile terminal, such as a wideband code division multiple access (WCDMA) system.
In the description of the preferred embodiments of the present specification the terms received signal code power (RSCP) and interference signal code power (ISCP) will be used to refer to the RSCP and ISCP values calculated from the received symbols.
FIG. 1 depicts a base station 10 in communication with a mobile terminal 12. In this example the base station 10 is transmitting a signal 14 in the downlink direction to the mobile terminal 12. The downlink physical channels include a common pilot channel (CPICH) and downlink dedicated physical control channel (DL DPCH). The signal to interference ratio (SIR) of the radio link 14 between the base station 10 and the mobile terminal 12 can be calculated as the RSCP divided by the ISCP. In most systems, however, the ISCP measurements typically have very high variation, which can lead to poor power control performance. The largest source of this variation is typically the sampling error because of the small number of symbols available for each calculation. Much less of the variation of the measured ISCP is due to true variations of the ISCP.
To reduce the variation of the ISCP caused by the sampling error, and to thereby improve the accuracy of SIR measurements, when calculating the SIR in mobile terminal, the ISCP will typically be smoothed, for example, with an Infinite Impulse Response (IIR), or any other low pass filter. This smoothed ISCP (Smoothed_ISCPt) can be calculated as follows:Smoothed_ISCPt=λ·Smoothed_ISCPt-1+(1−λ)·ISCPt and the Signal to Interference Ratio (SIR) can therefore be expressed as follows:
      S    ⁢                  ⁢    I    ⁢                  ⁢    R    =            R      ⁢                          ⁢      S      ⁢                          ⁢      C      ⁢                          ⁢              P        t                    Smoothed_ISCP      t      where λ is a coefficient that determines how much smoothing is applied.
However, a problem can occur when there is a large component of common variation between the measured RSCP and ISCP. This problem arises because the smoothed ISCP lags the true ISCP, but the RSCP adjusts instantly, which causes an offset in the measured SIR. Thus there is a trade off between the benefits of smoothing the ISCP and the drawback of introducing an offset to SIR measurements. This trade off is particularly difficult to manage when the RSCP and ISCP change rapidly, e.g. fast fading conditions, and in these circumstances the performance of inner loop power control may be poor.
The inventor has determined that these problems can be addressed by reducing the amount of common variation between the RSCP and ISCP. More particularly, the inventor has determined in certain systems a reference that can be found which can be used to measure and/or remove the common variation of the measured RSCP and ISCP. In this case the SIR can be calculated as follows:
                    S        ⁢                                  ⁢        I        ⁢                                  ⁢        R            =                                    R            ⁢                                                  ⁢            S            ⁢                                                  ⁢            C            ⁢                                                  ⁢                          P              t                                            C            t                                    Smoothed_ISCP          t                      ,                  ⁢    where              Smoothed_ISCP      t        =                  λ        ·                  Smoothed_ISCP                      t            -            1                              +                                    (                          1              -              λ                        )                    ·                                    I              ⁢                                                          ⁢              S              ⁢                                                          ⁢              C              ⁢                                                          ⁢                              P                t                                                    C              t                                      ⁢                                  ⁢        and            Ct is a reference for the component of common variation between the RSCP and the ISCP. Preferably this reference is not derived from the RSCP or the ISCP of the same channel.